stm32/ADC: Fix prescaler calculation to include max frequency.

Due to the integer rounding rules one has to subtract 1 from the numerator.
For example:

Let max clock be 55 and supplied clock be 110
110/55 = 2 which results in the divider being set to 4 and the clock after division ends up being 27 instead of 55
Subtracting 1 to the numerator get around the rounding issue
109/55 = 1 which results in the divider being set to 2 and the clock after division ends up being 55 which is exactly max clock

5 files changed, 10 insertions, 5 deletions

diff --git a/embassy-stm32/src/adc/adc4.rs b/embassy-stm32/src/adc/adc4.rs
index 255dc7956..1302dffb8 100644
--- a/embassy-stm32/src/adc/adc4.rs
+++ b/embassy-stm32/src/adc/adc4.rs
@@ -128,7 +128,8 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        // Calculate prescaler in a way where the clock can hit MAX CLK
+        let raw_prescaler = frequency.0.saturating_sub(1) / MAX_ADC_CLK_FREQ.0;
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,
diff --git a/embassy-stm32/src/adc/c0.rs b/embassy-stm32/src/adc/c0.rs
index f2837a8f1..bd9a3e2c6 100644
--- a/embassy-stm32/src/adc/c0.rs
+++ b/embassy-stm32/src/adc/c0.rs
@@ -66,7 +66,8 @@ pub enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        // Calculate prescaler in a way where the clock can hit MAX CLK
+        let raw_prescaler = frequency.0.saturating_sub(1) / MAX_ADC_CLK_FREQ.0;
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,
diff --git a/embassy-stm32/src/adc/g4.rs b/embassy-stm32/src/adc/g4.rs
index 43498966f..ac0a6196f 100644
--- a/embassy-stm32/src/adc/g4.rs
+++ b/embassy-stm32/src/adc/g4.rs
@@ -72,7 +72,8 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        // Calculate prescaler in a way where the clock can hit MAX CLK
+        let raw_prescaler = frequency.0.saturating_sub(1) / MAX_ADC_CLK_FREQ.0;
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,
diff --git a/embassy-stm32/src/adc/v2.rs b/embassy-stm32/src/adc/v2.rs
index e94a25b24..57f252e13 100644
--- a/embassy-stm32/src/adc/v2.rs
+++ b/embassy-stm32/src/adc/v2.rs
@@ -71,7 +71,8 @@ impl Prescaler {
         // Datasheet for both F4 and F7 specifies min frequency 0.6 MHz, typ freq. 30 MHz and max 36 MHz.
         #[cfg(not(stm32f2))]
         const MAX_FREQUENCY: Hertz = Hertz(36_000_000);
-        let raw_div = freq.0 / MAX_FREQUENCY.0;
+        // Calculate prescaler divider including MAX_FREQ
+        let raw_div = freq.0.saturating_sub(1) / MAX_FREQUENCY.0;
         match raw_div {
             0..=1 => Self::Div2,
             2..=3 => Self::Div4,
diff --git a/embassy-stm32/src/adc/v4.rs b/embassy-stm32/src/adc/v4.rs
index b66437e6e..c68684cb2 100644
--- a/embassy-stm32/src/adc/v4.rs
+++ b/embassy-stm32/src/adc/v4.rs
@@ -93,7 +93,8 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        // Calculate prescaler in a way where the clock can hit MAX CLK
+        let raw_prescaler = frequency.0.saturating_sub(1) / MAX_ADC_CLK_FREQ.0;
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,